Shifting rivers in India's largest tea producing state and abnormally high rainfall this year is destroying hundreds of acres of tea gardens and could cut output in the world's second-largest tea grower.

More than a tenth of the 18,000 hectares of plantations, or tea gardens, in India's northeast state of Assam could be washed away as the mighty Himalaya-born Brahmaputra and other smaller rivers flood the region where century-old operations grow over half of India's tea.

"Some tea gardens have already fallen into rivers and some of them are on the verge of disappearing," said Dipanjol Deka, secretary general of Tea Association of India (TAI) in Guwahati, the main city in the region.

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"In the long run there is a possibility of production loss and overall loss to the industry."

India consumes the bulk of its tea production. Last year, it exported a fifth of its 979 million kilograms (kg) of tea output, earning about $570 million.

Tea has been commercially grown in Assam since the early 19th century, after the East India Company which governed British possessions in the subcontinent lost its monopoly on tea-trade with China.

Assam's 850 gardens employ over 800,000 people and export the strong tea to over 80 countries including Russia and Britain. In 2009 the state produced nearly 500 million kg of tea.

But annual summer floods in the state which receives heavy monsoon rains has led to rivers breaking banks and wearing away slopes of gardens where tea is grown. This year, the region has received more rain than usual, weather officials say.